1. Field of the Invention
This invention relates to a focus detecting system in an optical device such as a camera, and more particularly to a focus detecting system which uses the imaging light beam from an imaging optical system in, e.g., a single lens reflex camera, to effect focus detection.
2. Description of the Prior Art
As a focus detecting system of a camera, there is known one in which the exit pupil of the imaging optical system is divided into two and the amount of deviation is observed from the relative positional relation between two object images formed by the light beams passed through the divided exit pupils, whereby the state of focus is discriminated. For example, a system in which a fly-eye lens group is disposed on the predetermined imaging plane (image-forming plane) of the imaging optical system of a camera and two deviated object images are created correspondingly to the amount of defocus of the imaging optical system is disclosed in U.S. Pat. No. 4,185,191. Also, so-called secondary imaging process in which the object images of aerial images formed on the predetermined imaging plane by two juxtaposed secondary imaging systems are directed to a solid state image sensor surface to detect the relative positional deviation of the object images is disclosed in Japanese Laid-open Patent Application Nos. 118019/1980 and 155331/1980. The latter, i.e., the secondary imaging process has the advantage that it requires no special optical system, although the full length of the optical system thereof is somewhat great.
The principle of the focus detecting device of the secondary imaging process will hereinafter be described briefly by reference to FIG. 1 of the accompanying drawings. A field lens 2 has the same optical axis as an imaging optical system 1, and two secondary imaging lenses 3a and 3b are juxtaposed rearwardly of these, and photoelectric conversion element arrays 4a and 4b are further disposed rearwardly of the lenses 3a and 3b. Designated by 5a and 5b are stops provided near the secondary imaging lenses. The field lens 2 substantially, images the exit pupil of the imaging optical system 1 on the pupil planes of the two secondary imaging lenses 3a and 3b. As a result, light beams to be incident on the secondary imaging lenses 3a and 3b respectively are those emergent from two areas of equal dimensions, which never overlap each other and which correspond to the secondary imaging lenses 3a and 3b, on the exit pupil plane of the imaging optical system 1. When object images as the aerial images formed near the field lens 2 are re-imaged on the surfaces of the photoelectric conversion element arrays 4a and 4b by the secondary imaging lenses 3a and 3b, the positions of the two re-imaged object images are variable in accordance with the changes in the poistion along the direction of the optical axis whereat each object image is formed. FIGS. 2A-2C of the accompanying drawings show the manner in which such phenomenon occurs. In each of the near-focus state and the far-focus state shown in FIGS. 2B and 2C, respectively, relative to the in-focus state of FIG. 2A, the two object images formed on the surfaces of the photoelectric conversion element arrays 4a and 4b are displaced in opposite directions on the surfaces of the photoelectric conversion element arrays 4a and 4b and become deviated from each other. If the intensity distribution of each object image is photoelectrically converted and the relative positional deviation of the two object images is detected by the use of an electrical processing circuit, the state of focus can be discriminated.
As a method of processing the photoelectrically converted signal, for example, Japanese Laid-open Patent Application No. 87222/1979 corresponding to U.S. Pat. No. 4,230,401 and U.S. Pat. No. 4,250,376 are known. When the received light signals obtained by photoelectrically converting the object images, as the two secondary images, are denoted by a(i) and b(i) (i=1 through N), respectively, ##EQU1## is calculated either by an analog processing circuit or digitally with respect to a suitable constant k in the aforementioned known examples, and the direction of displacement of the imaging optical system 1 to be executed is determined by the positive or the negative of the valve of this V. In this case, the value of k is usually selected to k=1. In FIG. 3 of the accompanying drawings, characteristics A and B shown the light intensity distributions of the two positionally deviated object images to be photoelectrically converted. In FIG. 3, the abscissa represents the lengthwise direction of the photoelectric conversion element array and the ordinate represents the output values of the elements. The sets of points such as 41 and 41', 42 and 42', and so on represent that they are the output signal values of the corresponding element pairs of the photoelectric conversion element arrays 4a and 4b and, for example, in case k=1, the first term of equation (1) has the correspondence relation between the sets such as 41 and 42', 42 and 43', and so on and is the result of the addition of the absolute values of the differences between the signal values from the respective elements. Accordingly, this signal processing method is nothing but to find an area corresponding to the difference between characteristics A and B obtained from the two object images and to discriminate in which direction of the displacement of the lens this area portion concentrates more.
However, the above-described operation makes it necessary to find the absolute values and suffers from the disadvantage that the electric circuit for realizing this becomes complex. Also, subtractions, between the outputs of the individual element pairs are carried out and therefore, the errors of the output values are accumulated and the accuracy of the operation becomes a problem.
Also, in the operation processing method based on equation (1), the direction in which the imaging optical system 1 is to be displaced is merely discriminated. Therefore, in a focus detecting system wherein the state of focus is discriminated from the amount of deviation of two object images, a method is known which uses the relation that the amount of deviation and the amount of defocus of the two object images are substantially proportional to each other, to displace one object image relative to the other object image, thereby calculating the amount of displacement of the imaging optical system 1. This method has already been proposed with respect to the base-length range finder type focus detecting system. Also, the reduced cost of semiconductor integration elements has made it possible to process a considerable amount of information within a camera and thus, several operation processing methods using the above-described principle have been proposed with respect also to the TTL (Through-The-Lens) type focus detecting system which requires high accuracy. For example, in Japanese Laid-open Patent Application Nos. 75607/1981 and 45510/1982, corresponding respectively to U.S. Pat. Nos. 4,387,975 and 4,333,007, the object image represented by b(i) is changed in position relative to the object image represented by a(i) and as the degree of correlation representative of the degree of coincidence between the two object images, V of equation (1) is calculated by the circuit processing. That is, ##EQU2## The amount of relative displacement m having V(m) set is repetitively calculated with respect to each integer value of the range m.sub.1 .ltoreq.m.ltoreq.m.sub.2. A graph in which the value of V(m) has been plotted relative to m is as shown in FIG. 4. When the two object images have become coincident with each other, V(m) should be 0 and thus, in FIG. 4, there is an amount of deviation of the images corresponding to 1.5 picture cell.
However, the non-linear processing of absolute value operation is included in equation (2) and this appears as an increase in operation step in the digital processing using a microcomputer or the like. Also, overlap of noise components occurs in such operation process taking the difference between two signals and this causes reduction in noise-resistant property.